A sensor 100 having a membrane structure according to a prior art is shown in FIGS. 8A and 8B. The sensor 100 is a thermopile type infrared sensor, and disclosed in Japanese Unexamined Patent Application Publication No. H04-98883. The sensor 100 has a thin film membrane 112. A hot contact point 115a of a thermocouple 115 (i.e., a hot junction 115a of a thermocouple 115) is disposed on the membrane 112 so that thermal separation between the hot contact point 115a and a cold contact point 115b of the thermocouple 115 (i.e., a cold junction 115b of the thermocouple 115) is improved.
In the above sensor 100, an upper surface of the sensor 100 disposed on the thermopile 115 is uneven, i.e., the upper surface has a wavy structure, as shown in FIG. 7. On the other hand, the other surface of the sensor 100 (not shown), where the thermopile 115 is not formed, is even, i.e., flat. By means of the wavy structure, stress is concentrated in this uneven portion.
When comparatively large stress is applied to the sensor 100 by means of thermal stress or distortion of the sensor 100 in a case of manufacturing process or a case of operating the sensor, the membrane 112 easily cracks because the mechanical strength of the membrane 112 is comparatively weak. Further, the membrane 112 may be broken by the large stress.
It is considered that thickness of a film for providing the membrane 112 becomes thicker so as to protect the sensor from cracking or being broken. However, in accordance with becoming thicker, thermo-conductivity of the film becomes large, so that thermal separation between the hot contact point 115a and the cold contact point 115b is deteriorated. Therefore, a sensitivity of the sensor is decreased. Further, it is considered that material of the film is changed to new material, which has comparatively low thermo-conductivity, so as to compensate the deterioration of the thermal separation. However, this makes the manufacturing cost increase.